Parabolic reflector antenna

ABSTRACT

A PARABOLIC REFLECTOR ANTENNA HAVING A FEED SYSTEM WHICH IS AN INTEGRAL PART OF THE REFLECTOR CONTOUR WITH THE FOCAL LINES OR FOCAL CIRCLES BEING COINCIDENT WITH THE LOCUS OF POINTS THAT COMPRISE THE REFLECTOR CONTOUR.

Jan. 5, 1971 A. A. ONDREJKA` '3,553,705

` yARABoLIC REFLEToR ANTENNA Filed Aug. 12,`196S 5 Sheets-Sheet. 1

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Jan. 5,'1971 A. A. ONDREJKA 3,553,705

" PARABOLIC REFLECTOR ANTENNA Filed Aug. l2, 1968 v 5 Sheets-SheetI 4.

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` VPARABoLIc REFLECTOR ANTENNA Filed Aug.r 12, `1968 'xsneets-sheet 5wlw/pferd@ United States Patent O Int. Cl. H01q 19/12 U.S. Cl. 343-771 5Claims ABSTRACT OF THE DISCLOSURE A parabolic reflector antenna having afeed system whichis an integral part of the reflector contour with thefocal lines or focal circles being coincident with the locus .of pointsthat comprise the reflector contour.

BACKGROUND OF THE INVENTION :the intersection of two identical paraboliccurves so that the focal points of the parabolic curves are coincidentwith the locus of points that compose the antenna crosssection. Thisantenna reflector cross-section is a parabola which isa uniqueconfiguration different from the ordinary parabolas with external focalpoints. This parabola can be projected or expanded into a cylindricaltype revolution by rotation.

A type ofv communications or radar antenna used by the military andcommercial agencies is known as the lparabolic reflector antenna. Thistype may be divided into two basic classes: `the parabolic cylindricaland the paraboloid of revolution. These antennas with their parabolic4cross-sections have the property that electromagnetic rays lradiatingfrom the feed, located at the focal point, are reflected from theantenna and are transformed into a `plane wave.

The present parabolic cylindrical antennas and paraboloids of revolutionantennas use an external feed usually lllocated at lthe focal line orfocal point. A parabolic cylindrical antenna uses what is called astraight line source feed consisting of a series of dipoles or a slottedwaveguide located on the focal line of the parabolic reflector. Simpleflared beam patterns with good directivity are obtained. Theparaboloidal reflector antenna uses a ,point source fixed at the focalpoint so that the reflector takes the divergent rays from the pointsource and converts them into a beam of parallel rays. These raysproducea highly directive pattern known as a pencil .fbeam. l

Since the present parabolic reflector antennas use external feeds thathave to ybe located at an external focal point or focal line, they mustbe supported by some struc- ,tural means, such as struts, braces, or insome cases by Vthe inherent structural strength of the waveguide. Beingl in front4 ofthe antenna reflector, the feed system consisting ofdipole or horn, plus waveguide and support structures such as struts, orbraces cause aperture blockage vwhich reduces antenna efficiency. Thewind blowing lon the feed system may cause distortion of antennareflector contour, or shift the feed from the focal plane Acausing largesidelobes and perturbations in the phase ofthe aperture distribution.The focal point fed parab- -oloid when'the pointed at high elevationangles, has a high level spillover of energy which illuminates a hotearth and results in an unusually large antenna noise contribution.Spillover around the paraboloid causes an increase in antenna bytranslation, or into a special paraboloid of 3,553,705 Patented Jan. 5,1971 rice back lobe intensity to the detriment of the main lobeintensity. If the feed support structure sags due to gravity or ice,beam pointing errors will occur. Thus, the invention is superior to thepresent designs in that it will eliminate or greatly reduce theweaknesses of the existing parabolic antennas.

SUMMARY OF THE INVENTION In accordance with the invention, a uniqueparabolic reflector antenna is provided that has a feed system which isan integral part of the reflector contour. The derived equations ofspecial parabolic curves enable the focal lines or focal circles to becoincident with the locus of points that comprise the reflector contour.Since the focal lines or focal circle are an integral part of thereflector contour, there can be obtained :a cylindirical parabolicreflector antenna or a paraboloidl of revolution reflector antenna withno aperture blockage, increased efficiency, decreased spillover energy,and reduced side and back lobes.

The concept of having a parabolic reflector antenna with a focal line orfocal circle that is an integral part of the reflector contour is aprime feature of this invention. This enables the feed system to be anintegral part of the reflector contour, thus leaving a free unrestrictedaperture for the reflection or impingement of electromagnetic energy.Therefore, all projections, outriggers, supports, feed horns, andwaveguides that block the conventional parabolic reflector apertures areeliminated.

It is an object of this invention to provide a parabolic reflectorantenna having a feed system which is an integral part of the reflectorcontour.

Another object of this invention is to provide a parabolic reflectorantenna whose cross-section is created from the intersection of twoidentical parabolic curves so that the focal points of the paraboliccurves are coincident with the locus of points that compose the antennacrosssection.

Still another object of this present invention is to provide a paraboliccylindrical reflector antenna whose antenna efliciency is increased byan unblocked aperture since the source feed and focal line are anintegral part of the antenna reflector contour.

Yet another object of the invention is to provide a parabolic reflectorantenna with reduced side lobes and back lobes since the source feedsystem is an integral part of the` antenna reflector and thus anyreflections or scattering of rays resulting fromrimpingement on thefeedrsystem are held to a minimum.` l

A further object of the invention is to provide a parabolic reflectorantenna with reduced energy spillover since the feed system is inset inthe antenna contour most of the energy is directed towards thereflector, thus reducing noise and blacklobe intensity.

A still further object of the invention is topprovide a parabolicreflector antenna with reduced reflector surface deviation anddistortion since wind loads and ice loads will be reduced as the feedsystem is not exposed. The integral feed could serve as a lstructuralmember, actually stiffening and strengthening the reflector. The effectsof gravity will be greatly nullifled, and defocusing will not -be aproblem as in the standard parabolic antennas.

Still yet another object of Vthis invention is to provide a parabolicreflector antenna to obtain greater gain `and reduced side lobes andcontrolled directivity of beam patterns since in the paraboloidalantenna extremely sharp pencil beam patterns can be obtained.

Another object of this invention is to provide a para bolic reflectorantenna design that can be built in different sizes dependent on thefrequency desired.

These and other objects, features, and advantages, will become moreapparent from the following description taken in conjunction with theaccompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a view of the two identicalintersecting parabolic curves, with parallel axes, and the creation ofthe unique parabola curve;

FIG. 1B shows pattern of rays in relationship to focal points;

FIG. 1C is a view of the two identical intersecting parabolic curveswith perpendicular axes producing the unique parabolic curve;

FIG. 1D shows pattern of rays in relation to focal points;

FIG. 2 is a wiew of a parabolic cylindrical reflector antenna, with aseries of dipoles in a straight line feed system;

FIG. 2A is a partial view of an inset slotted waveguide feed system;

FIG. 2B is a partial cross-sectional view of antenna reflector and aninset waveguide with another alternate feed system utilizing waveguidehorns projecting from the waveguide;

FIG. 3 is a cross-sectional view of a paraboloidal antenna;

FIG. 3A is a front view of a paraboloidal antenna;

FIG. 3B is a partial view of the feed system for FIG. 3; and

FIG. 3C is a back view of a paraboloidal antenna.

Referring now to the FIGS. 1A-1C showing the essence of the invention,the special parabolic curve resulting from the intersection of twoparabolas.

In FIG. 1A the parabolas labeled as 11 and 12 are plotted on twodifferent rectangular coordinate systems so that the parabola axes areparallel, and the ordinate separation is 2a; so that the focal point ofparabola 12 falls on parabola 11 and the focal point of parabola 11falls on parabola 12. The letter a is identified as the focal distancefrom the vertex of the parabola, and also the distance from the vertexof the parabola to the directrix measured on the parabola axis. It is tobe noted that the x and x1 axes are the axes of the parabolas. The y andy1 axes are coincident. The equation for a parabola with its axiscoincident with the x axis is where a is the focal distance from thevertex. We are concerned with the special parabolic curve (f"f2)resulting from the intersection of the upper arm or limb of parabola 11and the lower arm or limb of parabola 12. Since the two coordinatedsystems are 2a apart on the ordinate axis, the equation for the upperlimb of parabola 11 is To find the coordinates of the vertex of thisspecial parabola, solve for x by eliminating y..

Therefore, the coordinates of the vertex of the parabola are x=a/ 4, y=ain the x, y coordinate system.

Using the x, y coordinate system, we can plot the upper limb of theparabola using the equation y=\/4ax, and the equation y=-\/4ax+2a forthe lower limb. It is to be noted that in the x, y coordinate systemthat the coordinates of one focal point (f1) are a, O, and thecoordinates of the other focal point (f2) are a, 2a.

On the other hand, if we desire to have one equation for the parabola byincorporating a new coordinate system x", y so that the x axis coincideswith the axis of the parabola and the y axis goes through the vertex 0,we come up with the equation 5": i wm'- a] and thus the coordinates offocal point 1 (f1) are (a-a/ 4), -a and the coordinates for focal point2 are (fa-le) In FIG. 1B, there is shown the parabola taken from FIG.1A. It shows that the axes of parabola 11 and parabola 12 are parallelto each other and they are spaced a distance 2a apart. Note that theupper limb of parabola 11 and the lower limb of parabola 12 are mirrorimages of each other. The axis of the parabola is equidistant fromidentical mirror points of each limb. Note the way the parallel raysreflect from the limbs of the parabola and cross the parabola axis andimpinge on the focal points, when the rays are approaching the parabola.Rays emitted from each focal point cross the axis and are reflected fromthe parabola into parallel rays. It must be remembered that in acylindrical reflector antenna that there are two focal lines, coplanar,and located equidistant a on each side of the axis. This cylindricalreflector antenna configuration is obtained by translating the parabolain a direction perpendicular to the plane of the parabola so that theaxis becomes a plane.

To obtain a paraboloid of revolution, rotate the parabola around theaxis as per (Theorem of Pappius or Guldinius). The focal points of theparabola are then a part of a focal circle. The rays produce a highlydirective pattern known as a pencil beam. v

FIG. 1C shows another form of the parabola where the intersectingparabolas have their axes perpendicular to each other. The basicequation used to derive the parabola is .x1/2 -iy`1/2=l1`1/2 where b isa constant, which gives you a parabola whose axis is inclined 45 to thex, y axis, and where the focal point coordinates are (Ia/2, b/2). Y

This is shown as parabola 11. Its mirror image, Whose equation isy=2\/I2x, is moved down a distance bV-b, so that the focal point ofparabola 12 falls on parabola 11, and the focal point of parabola 11falls on parabola 12. By transferring the coordinate axes so that theorigin is at the vertex of the parabola, we have the axis (x) goingthrough the vertex at a distance c from. the x axis. The y axis is at adistance D from the y axis. The equation for this parabola is wherehngt/Mazda b==a constant In FIG. 1D there is shown the parabola takenfrom FIG. 1C. It shows that the axis of parabola 11 and parabola 12 areperpendicular to each other so that the axis is the bisector of theangle between the axes. Note the way the rays emitting from the focalpoints are reflected from opposite limbs of the parabola and becomeparallel. These parallel rays combine and become beams. Note that thebeams are perpendicular to each other.

` It must be remembered that in a cylindrical reflector antenna thatthere are two focal lines, coplanar, and located equidistant from theaxis. This cylindrical reflector antenna configuration is obtained bytranslating the parabola in a direction perpendicular to the plane ofthe parabola so that the axis becomes a plane. The pattern of thisantenna will be two flared directive beams perpendicular to each other.

To obtain a paraboloid of revolution, rotate the parabola around theaxis as per Theorem of Pappius or Guldinius. The focal points of theparabola are then a part of a focal circle. The rays produce a hollowcone beam.

FIG. 2 is a view of a parabolic cylindrical reflector antenna, and sinceits cross-section is a parabola, may be referred to as a paraboliccylindrical reflector antenna. Waveguide is set below the surface ofantenna contour 21 so that the series of dipoles 22 are on the focallines 23 of the antenna. Parasitic dipoles 24 shape and direct the raysas they are emitted or impinge on the dipoles 22. Plastic strip 25 isinset into the antenna 26 so that the parabolic contour is smooth and itprotects the dipoles from Wind and ice loads. The strip has cutouts thatallow the dipoles to come through so that the top of the strip 25 andthe tops of the dipoles 22 and 24 are in the same curved plane of theparabolic surface. Structural framework 27 is used to hold the reflectorantenna on a pedestal not shown.

FIG. 2A shows a partial view of another feed system for the cylindricalparabolic reflector antenna. Waveguide 30 includes slots 31 that are cutthrough one surface of the waveguide 30. Length of slots, spacing, andangularity depend on the frequency band that the antenna is designedfor. Plastic strip 34 is bonded to the waveguide 30 and keeps out thedirt and allows pressurization of waveguide. Plastic may be atransparent type, or may be made of reinforced plastic such asfiberglass. Waveguide 30 is a straight run without any joints, exceptfor flanges at its extremities. Waveguide 30 is inset into the antennareflector 36 that is properly rigidized by braces, channels, and anglesnot shown. Right angle flanged joint 35 fastened to the end of waveguide30 on an end, and may have a flexible waveguide fastened on the otherend which leads to the associated transmitter and receiver.

FIG. 2B shows a partial cutaway view of the side of reflector 36 of FIG.2A with another type of feed system. This feed system consists of awaveguide 40 that has a series of small horns 41 fastened through oneside of waveguide 40 so that energy can be transmitted through thesehorns into or out of the waveguide. The dimensions of the horns andspacing depend on the antenna size, and frequency that the antenna isdesigned to operate at. Inset plastic strip 44 serves to protect thehorns and `waveguide from the weather. Structural members 42 are anglesor channels that support waveguide 40 and rigidize antenna 46.

Other types of feed systems used in present antenna designs may beadapted to work with the parabolic cylindrical reflector antenna. Itwill be apparent to one skilled in the art that various changes,alterations, modifications and substitutions can be made in thearrangement and location of the various elements without departing fromthe true spirit and scope of the invention.

FIG. 3 shows a cross-section of another paraboloid of revolutionreflector antenna. Antenna 56 is shown lwith its structural frame 57that is fastened to a pedestal not shown. Focal circle 58 locates thefeed system. This feed system can be any of the types shown in FIGS. 2,2A, and 2B, except that the waveguide is formed into a circularconfiguration (annular ring) as shown in FIG. 3C. Right angle waveguidejoint 59 is the junction for all of the waveguide leading from theassociated transmitter and receiver.

FIG. 3A is the front view of the paraboloid of revolution reflectorantenna of FIG. 3, showing the circular configuration (annular ring) ofthe plastic protective strip 54 and the focal circle 58.

FIG. 3B shows a closeup of a partial View of the feed system. 50 is thewaveguide, 52 is one of the dipoles inserted into waveguide 50, and 52Ais one of the parasitic dipoles that shape and direct the emission orimpingement of rays to or from the paraboloid of revolution. Plasticprotective annular strip 54 protects the dipoles from the weather. 57are the angles or channels that secure waveguide 50 and rigidize.` theantenna.

FIG. 3C is the backside of the paraboloid of revolution reflectiveantenna. Waveguide 60 is formed into an annular ring so that it followsthe path of the focal circle. Dipole antennas 62 are inserted into theside of waveguide fitting 69 connects the circular arm of waveguidedipoles are located on the hidden side of waveguide 60, they areindicated in phantom lines. Right angle waveguide fitting 69 connectsthe circular arm of waveguide 60 in one plane on one end, and joinsflexible waveguide 70 in another plane on the other end. The rest of thewaveguide plumbing lea-ding to the associated transmitter and receiver(that are not shown) is connected to the other end of the flexiblewaveguide. The spacing, shape, and size of the dipoles depend on thefrequency desired. Flanges 7l are bolted or clamped together to make theannular shape for the large antennas. On small antennas these flangejoints might be eliminated and the annular shape of the waveguide may beobtained by electro-forming it in one piece. Special termination fitting72 is closed at one end by plate 73 and that is filled with material 74that absorbs electromagnetic energy, and thus eliminates disturbinginternal reflections. If special shaped beams are desired, or ifmultiple phased. beams are desired, the annular waveguide could bedivided into sectors with individual feeds that could have variouspolarizations, be fed in phase, sequence, or in a cyclic manner. It isto be noted that emphasis has been placed on the two special cases forthe formation of the special parabola, first by the intersection of twoparabolas with axes parallel to each other, and the second case of theparabolas with axes perpendicular to each other forming the parabola.The special parabola can be formed where the parabola axes are inclinedat different angles to each other, as long as the focal points of eachparabolic curve fall on the opposite parabolic curve. From theforegoing, it will be seen that the invention has been presented withparticular emphasis on certain preferred embodiments. It will beapparent to one skilled in the art that various changes, alterations,modifications, and substitutions can be made in the arrangement andlocation of the various elements without departing from the true spiritand scope of the invention as defined in the claims.

What is claimed is:

1. A parabolic reflector antenna comprising a p-arabolic reflectorformed by intersecting the upper limb of a first parabola and the lowerlimb of a second parabola so that the focal point of said secondparabola falls on said upper limb ofsaid first parabola and the focalpoint y,of Said first parabola falls on said lower limb of said secondparabola with the axes of said flrst and second parabolas being paralleland separated a preselected distance, said eXas being focal lines forsaid parabolic rellector, and electromagnetic feed means positioned atsaid focal lines and being an integral part of the reflecting surface ofsaid parabolic reflector.

2. A parabolic reflector antenna as described in claim 1 wherein saidelectromagnetic feed means is comprised of a slotted waveguide for eachof said focal lines.

3. A parabolic reflector antenna comprising a parabolic reflector whosecontour is formed by intersecting the limb of a rst parabola with thelimb of a, second parabola so that the focal point of said secondparabola falls on said limb of said first parabola and the focal pointof said first parabola falls on said limb of said second parabola withthe axes of said first and second parabolas being perpendicular to eachother to form focal lines, and electromagnetic feed means located onsaid focal lines and being an integral part of the reflecting surface ofsaid parabolic reflector.

4, A paraboloid of revolution reflector antenna comprising a paraboloidof revolution reflector whose contour is formed by intersecting the limbof a first parabola and the limb of a second parabola so that the focalpoint of said second parabola falls on said limb of said rst parabolaand the focal point of said first parabola falls on the said limb ofsaid second parabola with the axes of said first and second parabolasbeing parallel to each other and separated a preselected distance fromeach other to provide a resultant Vparabola and then rotating the axisof the resultant parabola to provide a focal circle for said paraboloidof revolution reflector, and electromagnetic feed means positioned atsaid focal circle and being an integral part of the reflecting surfaceof said paraboloid of revolution reflector.

5. A paraboloid of revolution reflector antenna as described in claim 4wherein said electromagnetic feed means is comprised of a slottedwaveguide curved into a circle.

References Cited UNITED STATES PATENTS 1,341,674 6/1920 Rhodin 24U-41,371,739,800 12/1929 Patten et al. 240-41.35 2,160,853 6/1939 Gerhard etal. 343-840 2,471,284- 5/1949 Rea 343-840 3,039,098 6/1962 Bickmore343-771 3,365,720 1/1968 Kelleher 343-914 FOREIGN PATENTS 612,75611/1948 Great Britain 24U-41.37

ELI LIEBERMAN, Primary Examiner I U.S. Cl. X.R.

